Cytostatin is a natural antitumor inhibitor of cell adhesion to extracellular matrix, blocking adhesion of B16 melanoma cells to laminin and collagen type IV in vitro (IC50s = 1.3 and 1.4 μg/ml, respectively) and B16 cells metastatic activity in mice. It induces apoptosis of FS3 mouse fibrosarcoma cells (IC50 = 3.1 μg/ml). Cytostatin potently and selectively inhibits protein phosphatase 2A (PP2A; IC50 = 29 nM against the catalytic subunit), while having no effect against PP1, PP2B, or PP5.